linux-kernel - Re: [patch V2 08/10] timer: Implement the hierarchical pull model

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Message-ID: <alpine.DEB.2.20.1704191100340.1829@nanos>
Date:   Wed, 19 Apr 2017 11:03:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-pm@...r.kernel.org, Arjan van de Ven <arjan@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [patch V2 08/10] timer: Implement the hierarchical pull model

On Wed, 19 Apr 2017, Peter Zijlstra wrote:
> On Wed, Apr 19, 2017 at 10:31:08AM +0200, Thomas Gleixner wrote:
> > On Wed, 19 Apr 2017, Peter Zijlstra wrote:
> > > On Tue, Apr 18, 2017 at 01:11:10PM +0200, Thomas Gleixner wrote:
> 
> > > > +	}
> > > > +	/* Allocate and set up a new group */
> > > > +	group = kzalloc_node(sizeof(*group), GFP_KERNEL, node);
> > > > +	if (!group)
> > > > +		return ERR_PTR(-ENOMEM);
> > > > +
> > > > +	if (!zalloc_cpumask_var_node(&group->cpus, GFP_KERNEL, node)) {
> > > > +		kfree(group);
> > > > +		return ERR_PTR(-ENOMEM);
> > > > +	}
> > > 
> > > So if you place that cpumask last, you can do:
> > > 
> > > 	group = kzalloc_node(sizeof(*group) + cpumask_size(),
> > > 	                     GFP_KERNEL, node);
> > 
> > Hrm, that would allocate extra space for CPUMASK_OFF_STACK=n. I'll have a
> > look.
> > 
> 
> How so? Remember your structure will look like:
> 
> struct group {
> 	/* ... */
> 	unsigned long cpumask[0];
> };
> 
> Which, even for the CPUMASK_OFF_STACK=n case, is exactly 0 bytes of
> total storage.

-ENOTENOUGHCOFFEE